1. Field of the Invention
The present invention relates to a polyvinyl alcohol (hereinafter referred to as PVA) film having excellent properties which make it suitable for use in the packaging of chemicals such as agricultural chemicals, microbiocides, and the like.
2. Description of the Background
A method of unit-dose packaging (unit pack) various chemicals such as agricultural chemicals under airtight packaging conditions in a water-soluble film has been recently employed, where the chemicals in packaged form are thrown into water so that the contents are dissolved or dispersed in water. The unit pack is advantageous in that users do not directly touch harmful chemicals, the measurement of chemicals for use in unnecessary as the unit-dose is already packaged, and the disposal of the containers and bags used for packaging chemicals is unnecessary or is easy.
Partially-hydrolyzed PVA films have been used in the past as water-soluble films for a unit pack. The conventional water-soluble films exhibit excellent cold water-solubility and mechanical strength. However, some types of chemicals, which are packaged in these films, cause problems by lowering the water-solubility of the films with the passage fo time during storage to the point that the films finally become water-insoluble or hardly soluble in water.
Chemicals which cause partially-hydrolyzed PVA films to become water-insoluble include, for example, alkali chemical substances, acid chemical substances, chlorine-containing chemical substances, chemical compounds of a salt of a polyvalent metal such as copper, cobalt, or the like, polycarboxylic acid containing chemical substances, polyamine-containing chemical substances, boric acid-containing chemical substances, agricultural chemicals such as insecticides, herbicides, and the like, and microbiocides.
Films for packaging alkali chemical substances which are known include films of PVA modified with salts of carboxylic acids. A film which is resistant to boric acid is a sorbitol-containing PVA film.
Known films for packaging acid chemical substances, chlorine-containing chemical substances (microbiocides, etc.) and agricultural chemicals include films fo polyethylene oxide and cellulose, However these films are defective in that their water-solubility is extremely low, their mechanical properties are poor, and their impact resistance at low temperatures is extremely low because they are hard and brittle.
Films which are known for packaging agricultural chemicals and microbiocides include water-soluble films that are prepared from PVA or carboxylate-modified PVA, wherein the polymer is combined with an auxiliary such as polyethylene glycol or propyl gallate. (See Japanese Patent Application Laid-Open No. Sho-53-24351 (1978).) Also water-soluble films for packaging agricultural chemicals are known, which are prepared from sodium 2-acrylamido-2-methylpropanesulfonate-modified PVA having added thereto a phenol derivative such as bisphenol A. (See JP Hei-7-118407 (1995).) However, in the event chemicals such as acid chemical substances, chlorine-containing chemical substances, agricultural chemicals and microbiocides are packaged in the conventional PVA films and stored for a long period, the PVA films lose their function as a water-soluble film, because they become discolored (a brown color) and become insoluble in cold water and hot water. A need continues to exist for a PVA based packaging film which is resistant to various chemical materials which are stored therein.